The present invention relates to an improved radiographic grid for use in an x-ray apparatus, especially for use in an x-ray mammography apparatus. More particularly, it relates to a radiographic grid having foil disposed about individual lamella for reducing lamellae density artifacts.
It has been well known since the early days of radiography that secondary or scattered x-rays reduce the contrast of an x-ray image. The low difference in x-ray absorption characteristics between cancerous and non-cancerous tissue has made mammography particularly susceptible to imaging problems caused by scattered radiation. A conventional Bucky grid, consisting of a series of lead foil strips separated by strips of x-ray semi-transparent spacers, helps remove scattered radiation from radiographic fields.
The thin strips of x-ray radiation absorbing material are called lamellae and are substantially aligned with the incident course of the radiation from the x-ray source, with the x-rays being transmitted through the gaps between the lamellae. The grid is positioned between the object being analyzed and the image receptor (or film) to reduce scattered radiation, thereby improving image contrast on the film.
Radiographic grids have been subject to various recent improvements. For example, U.S. Pat. No. 4,901,335 to Ferlic et al. teaches a reciprocating grid having at least a 90% open area at all positions of travel to transmission of directly incident x-ray radiation (i.e. radiation perpendicular to the tangent of the direction of travel of the grid at the point of incidence). The lamellae are individually positioned and aligned with respect to each other in a grid housing and then a cover sheet, substantially covered with an adhesive, is pressed down onto the edges of the lamellae.
Radiographic grids have proved to be a highly useful tool for removing scattered radiation from radiographic fields. However, it can be demonstrated that x-ray images produced with radiographic grids contain a "straight line" density artifact apparently associated with the lamellae. These lines correspond to the distance between individual lamella. Obviously, these line-shaped densities are undesirable as the goal with mammography or any other x-ray application is to eliminate all density related noise so that the resulting image is a true depiction of the patient's status.
Primary radiation is orientated in the same axis as the lamellae and passes between them to reach the film. Scattered radiation arises from many points within the patient, and is multidirectional, so that most of it is absorbed by the lamellae, and only a small amount passes between them. The lamellae line artifacts are not characteristic of prim or scattered radiation, but rather are subsequent to secondary radiation. The lamellae line artifacts are analogous to either a wave or a tertiary radiation.
The lamellae line artifacts appear resultant from or subsequent to the scattered radiation and are likely an additional emission from the lead or metal based lamellae. The basis for concluding that the lamellae line artifacts are tertiary radiation or wave-related is due to the observation that the lamellae emission which produces the density artifact occurs subsequent or resultant to the secondary radiation. Further, tests have shown that the emission appears to occur only in a downward direction (with respect to the position of the grid) as the lamellae line artifact does not materialize on film placed above the grid. Finally, tests have demonstrated that the line artifact is orientated with respect to the central ray of the x-ray source. In other words, the density line artifact is produced on the left side of lamellae positioned on the far right side of the film. Conversely, the line artifact density is produced on the right side of the lamellae positioned to the far left side of the film.
Regardless of whether the artifact originates from a third order energy source, this density, like all other densities associated with radiographic grids, should be eliminated. Little research has been done to find a solution. In fact, a search of standard references revealed no literature describing the effect.
A substantial need exists for a radiographic grid apparatus which eliminates the recently identified lamellae line artifact.